USSR in WWII, Allied superpower that bore the largest share of the land war against Nazi Germany on the Eastern Front, conducting major operations from 1941 to 1945 including the defense of Moscow, Stalingrad, Kursk, and the final advance into Berlin. Prior to the German invasion, the USSR had a agreement with Nazi Germany (Molotov–Ribbentrop Pact, 1939–1941), which coincided with Soviet territorial expansion in Eastern Europe, including the invasion of eastern Poland in 1939 and the Winter War against Finland (1939–1940), as well as annexations in the Baltic region and pressure on neighboring states.